1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as a copier or a laser beam printer and an electrophotographic image forming method.
2. Description of the Related Art
In order to ensure consistent print quality of image forming apparatuses such as copiers and laser beam printers, the conditions necessary to form an image, for example, a charging voltage and a light quantity of exposure, must be controlled so that the charge density of an electrostatic latent image (also referred to as a latent image) formed on the surface of an image bearing member is maintained at an appropriate value. Accordingly, after the surface of the image bearing member has been charged or exposed to light, the surface potential of the image bearing member is measured by a potential measurement device. The image forming conditions are controlled on the basis of the measurement result. Such a function is generally incorporated into image forming apparatuses. In some of these image forming apparatuses, two types of latent images are formed using a charging device and an exposure device, that is, a low-charge-density latent image obtained by charging the surface of an image bearing member and exposing the charged surface to light and a high-charge-density latent image obtained only by charging the surface of the image bearing member. Subsequently, the surface potentials of the formed latent images are measured. See, Japanese Patent Publication No. 62-10425. In order to obtain desired surface potentials of these latent images, the image forming conditions such as a charge intensity and an exposure intensity are controlled. Thus, consistent print quality is ensured. Herein, the term “charge density” means the amount of electrical charge per unit area.
In image forming apparatuses, it is better to use a potential measurement device capable of measuring the surface potential of an image bearing member in a noncontact manner. See, Japanese Patent Laid-Open No. 6-242166. If a potential measurement device is in contact with the surface of an image bearing member, the charge density of an electrostatic latent image formed on the surface of the image bearing member may be changed. Furthermore, charging characteristics of parts of the surface of the image bearing member may be changed due to the surface wear and the surface modification. As a result, normal potential measurement cannot be performed and consistent image quality cannot be ensured.
When potential measurement is performed, measurement errors occur because of various error factors. In order to perform accurate potential measurement, it is required to remove the causes of these error factors or correct these measurement errors. The error factors include aged deterioration that is the gradual change over time, for example, aged deterioration of charging characteristics of an image bearing member or aged deterioration of drive components of a potential measurement device. In the case of measurement errors caused by such error factors, the method of correcting measurement errors is effective. A potential measurement device employing this method has been disclosed. See, Japanese Patent Laid-Open No. 61-155863. In this potential measurement device, after exposure of the surface of an image bearing member has been performed so as to remove an electrical charge therefrom, the surface potential of the image bearing member is measured. The measurement value is stored in a storage unit as an offset. Subsequently, when the surface potential of a latent image formed on the surface of the image bearing member is measured, the offset is subtracted from the measurement value of the surface potential of the latent image. Thus, a measurement error is corrected.
In order to miniaturize an image forming apparatus, it is required to miniaturize an image bearing member that is a main component of the image forming apparatus. Referring to FIG. 6, with the miniaturization of an image bearing member 1, a charging device 2, an exposure device 3, and a developing device 5 are all disposed near the surface of the image bearing member 1, and are all disposed near a potential measurement device 4. Under such circumstances, it is difficult to accurately measure a surface potential of a latent image formed on the image bearing member 1. The charging device 2 and the developing device 5 each have a high potential source that disturbs an electric field near a measurement surface of the potential measurement device 4, thereby causing measurement errors. In particular, a high-charge-density area 11 between the charging device 2 and the exposure device 3 on the surface of the image bearing member 1 is the high potential source nearest to the potential measurement device 4 and may therefore be a factor responsible for the largest measurement error. Such a high-charge-density area that has not to be exposed to light yet is referred to as a “pre-exposed area 11,” hereinafter.
The degree of a measurement error that is caused by the pre-exposed area 11 and affects the surface potential measurement result of a latent image is determined on the basis of the following three factors.
The first factor is the distance between the pre-exposed area 11 and a measurement area 12 on the surface of the image bearing member 1 on which potential measurement is performed. More accurately, the first factor is the distance between an exposure position 13, which is placed at one end of the pre-exposed area 11 and is exposed to light by the exposure device 3, and a measurement area center point 14 directly below the measurement surface of the potential measurement device 4. If this distance is decreased for the miniaturization of the image bearing member, the value of a measurement error is increased. The reason for this is that if the above-described distance is decreased, the influence of an electric field generated from the pre-exposed area 11 on an electric field distribution near the measurement surface under the potential measurement device 4 is increased.
The second factor is a measurement distance 15 of the potential measurement device 4. The measurement distance 15 is the distance between the measurement surface of the potential measurement device 4 and the surface of the image bearing member 1. If the measurement distance 15 increases, the value of the measurement error is increased. The reason for this is that if the measurement distance 15 increases, the sensitivity of the potential measurement device 4 to an electric field generated from an electrostatic latent image formed on the surface of the image bearing member 1 is decreased, whereas the influence of an electric field radially generated from the pre-exposed area 11 is increased. In order to prevent this situation, the measurement distance 15 must be decreased. If the measurement distance 15 is decreased, however, a discharge occurs between the surface of the image bearing member 1 and the potential measurement device 4 during a period other than a potential measurement period. Consequently, there is the increased possibility that the surface of the potential measurement device 4 or the surface of the image bearing member 1 will be destroyed. Accordingly, from the viewpoint of the reliability of an image forming apparatus, the method of decreasing the measurement distance 15 cannot be employed.
The third factor is the difference between the surface potential of the measurement area 12 and the surface potential of the pre-exposed area 11. The larger this potential difference, the higher the value of a measurement error. The reason for this is that if the above-described potential difference is increased, the intensity of an electric field generated from the measurement area 12 becomes lower than that of an electric field generated from the pre-exposed area 11. That is, the influence of the electric field generated from the pre-exposed area 11 becomes relatively strong.
Among the above-described three factors, in the case of the first and second factors, the degree of an influence on a surface potential measurement result is determined in accordance with an element of internal space design of an image forming apparatus, for example, the size of the image bearing member 1, the exposure position 13, or the installation position of the potential measurement device 4. These space design elements are not markedly changed unless there is a change over time or a destructive change in the image forming apparatus. Accordingly, it can be determined that the value of the measurement error caused by the first or second factor is substantially constant over time. In order to reduce such a measurement error, a method can be employed of obtaining the value of a measurement error using an experimental method or a numerical analysis method, storing the obtained value as an offset, and subtracting the offset from a measurement value.
However, in the case of the third factor, the difference between the surface potential of the pre-exposed area 11 and the surface potential of the measurement area 12 is markedly changed in accordance with the charge density of the measurement area 12. Accordingly, the measurement error caused by the third factor is markedly changed in accordance with the charge density of the measurement area 12. In this case, the above-described method of subtracting an offset, which is a constant value, from a measurement value cannot be employed so as to achieve good measurement error reduction.